Applications of Hess’s Law (AQA A-Level Chemistry): Revision Notes

Example: How to Perform a Hess' Law Calculation

Step 1: Identify the relevant reactions and the enthalpy values (either formation or combustion).

Step 2: Set up a Hess Cycle, showing all the different routes for the reaction.

Step 3: Apply Hess' Law by equating the total enthalpy change of different routes, ensuring all arrows are in the correct direction.

Step 4: Perform the calculation by adding or subtracting the enthalpy values, treating them like vectors.

Example: Thermal Decomposition of Sodium Hydrogen Carbonate ($NaHCO₃$)

Hess' Law can be applied to the thermal decomposition of sodium hydrogen carbonate, which cannot be measured directly.

The reaction is:

Using Hess' Law, we can calculate the enthalpy change for this reaction by considering the formation enthalpies of sodium carbonate, carbon dioxide, and water, as well as sodium hydrogen carbonate.

Example: Hydration of Magnesium Sulphate ($MgSO₄$)

In the hydration of magnesium sulphate:

The enthalpy change for this process can be determined using Hess' Law by combining the enthalpy changes of formation for anhydrous magnesium sulphate, water, and hydrated magnesium sulphate.

Example: Hydration of Copper(II) Sulphate ($CuSO₄$)

For the hydration of copper(II) sulphate:

By applying Hess' Law, we can calculate the enthalpy change using the enthalpy of formation values for anhydrous copper sulphate, water, and hydrated copper sulphate.

Summary

• Hess' Law states that the enthalpy change of a reaction is independent of the route taken.
• This law is used to calculate enthalpy changes for reactions that cannot be directly measured.
• You can apply Hess' Law using either enthalpy of formation or enthalpy of combustion values.
• Common applications include the thermal decomposition of $NaHCO₃$ and the hydration of $MgSO₄$ and $CuSO₄$.